/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
    
        if(head == null || head.next ==null) return head;
        
        ListNode dummyhead = new ListNode(-1);
        ListNode dummy = dummyhead;

        dummy.next = head;  //potential unique element

        ListNode curr =head.next;

        while(curr!=null){

            boolean flag = false;

//going into this loop means there are duplicate elements             
            while(curr!=null && dummy.next.val == curr.val){
                flag = true;
                curr=curr.next;
            }
//loop runs till curr is at a unique element
            if(flag){
                dummy.next = curr;  //changing potential unique element
            }

            //we already had the right unique potential element
            else{
                dummy = dummy.next;
            }

            if(curr!=null)curr = curr.next;
        }

        return dummyhead.next;
    }
}